Packing boxes are sealing devices mechanically embodying static or semi-dynamic imperviousness concerning the movement of shafts or control rods between an internal element and an external element. When a movement occurs, this is in principle alternative on rotating or moving. Packing boxes are extremely numerous in industry and in particular in industries producing cocks or valves where each of these units possesses at least one of these devices for movement of the control rod.
Generally speaking, a packing box in accordance with FIG. 1 is formed of a packing 1 acting as a sealing lining and disposed in a packing box 2 hollowed in a solid element, such as a valve body 3, a rammer 4 for compressing the packing, tightening bolts for producing the force for compressing the packing 1, and a set of elastic washers 6 disposed in this instance coaxially to the rod of the bolts 5 between the tightening nuts 7 and a thrustor 8 of the rammer 4 so as to suitably adjust the force for compressing the packing 1. The control rod 9, which passes through the perforations of the rammer 4, its thrustor 8 and the body of the valve 3, also extends through the packing box 2 and the axial compression produced on the packing 1 results in causing an expansion of the packing 1 in a radial direction against the control rod 9, thus rendering imperviousness along the latter.
According to the type of industry or more specifically according to the working conditions governed by the nature of the fluid and the pressure and temperature levels, different types of packings 1 have been proposed, such as asbestos wire-based braids, carbon and graphite fiber braids, glass fiber braids, metallic fiber braids or superimposed expanded graphite rings or polymer rings of various compositions having a rectangular, triangular or V-shaped section.
The performances of a packing box vary as a large number of parameters are involved. The linings constituted by a stacking of expanded graphite or PTFE (widely known under the name of Teflon) rings therefore require that the packing box 2 is completely closed so as to prevent any leak occurring. In order to achieve this, the mechanical plays as regards the control rod 9 need to be reduced to a strict minimum, either by retightening the bores of the valve body 3 and rammer, or by disposing rings made of a hard material and with appropriate dimensions which is placed at the bottom of the packing box 2 and under the rammer 4. The packing 1 is then compressed between the rings in question, which requires that the packing box 2 be elongated.
The choice of packings is also limited to high temperatures.